Pull -Wimplicit-fallthrough into separate config

This config will be added to Crashpad's BUILDCONFIG.gn defaults rather
than forced into all common configs.

Bug: crashpad:309
Change-Id: If4e2c0256b4f5c93604d57b14caf28880d40f646
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/mini_chromium/+/1773504
Reviewed-by: Joshua Peraza <jperaza@chromium.org>
diff --git a/build/BUILD.gn b/build/BUILD.gn
index 1c5fb09..ca3bba3 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn
@@ -245,10 +245,6 @@
     ]
   }
 
-  if (mini_chromium_is_clang) {
-    cflags += [ "-Wimplicit-fallthrough" ]
-  }
-
   cflags += common_flags
   asmflags += common_flags
   ldflags += common_flags
@@ -272,6 +268,13 @@
   }
 }
 
+config("Wimplicit_fallthrough") {
+  if (mini_chromium_is_clang) {
+    cflags = [ "-Wimplicit-fallthrough" ]
+  }
+
+}
+
 config("win_console") {
   if (mini_chromium_is_win) {
     ldflags = [ "/SUBSYSTEM:CONSOLE" ]